Silicon is commonly the primary constituent material of most common semiconductor chip circuitry, and as a consequence, there exists a widespread interest in silicon-based technology, such as silicon-based sensors or silicon-based microelectromechanical systems (MEMS). Chemical sensors or chemosensors are small synthetic molecules that produce a measurable signal upon interaction with a specific analyte, and are used to determine the existence and/or concentration of a subject analyte without involving complicated analytical techniques or disturbance of the system being analyzed. Among other numerous uses, chemical sensors may be used for detecting explosives at security checkpoints or in the battlefield, monitoring pollutants in wastewater and quantifying contaminants in chemical compositions.
Like gunpowder, which is a mixture of carbon, potassium nitrate (i.e., saltpeter) and sulfur, porous silicon combined liquid oxygen or nitric acid with was known to combust when ignited with, for example, a flame or heat-producing impact. However, nitric acid is extremely corrosive, rendering it an unfavorable candidate as a chemical sensor in combination with silicon. Similarly, liquid oxygen lacks adequate portability for use in quick and easy on-site applications, insofar as it requires extremely low temperatures (˜77K) to maintain its aqueous state. Moreover, nitric acid and liquid oxygen detonate porous silicon spontaneously on contact and must therefore be kept separate from the porous silicon and added to the porous silicon only at the instant an explosion is desired. Also, since they are liquids, nitric acid and liquid oxygen must be added to the porous silicon to obtain the explosion via dripping, injection, or other method. Liquid delivery of these oxidants therefore poses a problem inasmuch as the liquid may evaporate when stored over a long period of time, it may be difficult to deliver the liquid on cue, and it is difficult to prevent the corrosive liquid from damaging other components of the device housing the explosive.
Thus, conventional silicon-based explosives lack practical qualities that would make such explosives viable for use in chemical sensing applications, especially those that require quick analysis and portability in the field.
Conventional silicon-based sensors have drawbacks for MEMS applications as well. MEMS devices are microscale mechanical devices. For example, MEMS may integrate mechanical elements—with sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.